1. Field of the Invention
The present invention relates to a rack and pinion type steering apparatus and, more particularly, to improvement of a rack guide thereof.
2. Description of the Prior Art
A rack and pinion type steering mechanism has been widely used for a steering apparatus of an automobile. In such steering mechanism, a rack guide (a support yoke) is disposed on the back side of the position engaging with a pinion shaft at the rack shaft so that the rack shaft is securely engaged with the pinion shaft. The rack guide is fitted into a guide bore provided at a rack housing in the perpendicular direction to the rack shaft and is supported so that the rack shaft is properly pressed against the pinion shaft. At this time, in order to apply the proper pressure against the rack shaft, there is provided the minimum space between the rack guide and the inner periphery of the guide bore.
In the steering apparatus as described above, the rack guide made of sintered alloy has been used. In such a steering gear the space formed between the rack guide and the guide bore produces big hammering noise. To avoid this disadvantage, the rack guide made of synthetic resin has generally come to be used and many different techniques have been proposed. For example in Japanese Utility Model Application Laid-Open No. 57-103267 (1982), a rack guide is disposed with plural streaks of annular elastic ribs in the outer periphery thereof. In this case, the annular elastic ribs are pressed and fitted into the guide bore so as to abut the inner periphery of the guide bore, as a result, the rack guide is unlikely to smoothly move in its axial direction. To complement this disadvantage, it is necessary to provide higher press load of a spring thereof. It also has a disadvantage that the elastic ribs are easily deformed when they are loaded from the pinion shaft.
In Japanese patent application Laid-Open No. 59-216764 (1984), a rack guide is formed of double cylinders and its outer cylinder, ellipse in shape, is pressed and fitted into a guide bore in the rectangular direction to the direction of reciprocation of the rack shaft. In this case the rack guide is also unlikely to smoothly move in its axial direction. Since the outer cylinder is connected to the inner cylinder through a rib which is disposed in the shorter radius side thereof and there is not provided a rib in the longer radius side, that is, in the direction of the force being exerted most by load of the pinion shaft, there is a problem that it lacks in rigidity.
As described above, different problems exist in known apparatus of the examples. To solve those problems, there has been proposed that a rack guide made of synthetic resin of the same shape and size as that made of sintered alloy be fitted into the inner periphery of a guide bore with the minimum space between them as in the same way as the conventional. When the rack guide made of synthetic resin is utilized, since the rate of thermal expansion is higher than that of the rack guide made of sintered alloy at high temperature atmosphere, the rack guide is so tightly fitted into the guide bore that the required pressure to maintain proper engagement of gears of the pinion shaft with the rack shaft is not obtained. Thus making disadvantages that the steering wheel rotates with less reacting force or is worn out and thermal creep is produced in the rack guide. In order to avoid such inconvenience, when wider space is provided in advance to cope with thermal expansion, with actuation of the rack shaft and pinion shaft at room temperature, the rack guide is moved within the guide bore in the direction of its radius, that is, in the perpendicular direction to the pressing direction of the rack guide, as a result, the pressing direction becomes unstable and the reacting force at steering is reduced.